Validation of psoriasis disability index (PDI) questionnaire Sinhala version

Psoriasis is a chronic inflammatory dermatosis with significant physical and psychological impact leading to negative influence on the quality of life among patients with psoriasis. Other than the disease characteristics many external factors could operate in South Asian context. Lack of a reliable disease-specific instrument prevents objective estimation and monitoring of disability in patients with psoriasis and hence we aim to validate assess the psychometric properties of the Sinhala version of PDI.

A cross-sectional study conducted at dermatology clinic at a tertiary care National Hospital in Sri Lanka. Patients with psoriasis and on therapy at least 4 weeks prior to enrollment, aged more than 18 years, were included while those with already diagnosed psoriatic arthritis and/or nail psoriasis alone without any skin involvement and generalized pustular psoriasis de novo were excluded. All patients were examined by dermatologist to obtain disease characteristics. The reliability was assessed by internal consistency using Cronbach's α and item-total correlation. Convergent validity was measured with the known groups.

Of 199 patients studied, the PDI Sinhala version showed Cronbach's α of 0.86 (all 15 items) and ranged from 0.57 to 0.77 for subscales. PDI score and Dermatology Life Quality Index (DLQI) showed good correlation of coefficient 0.76 (p < 0.01). Positive associations were noted with extent and severity of psoriasis when using sample medians (p < 0.05). The dimensionality of the PDI was determined using exploratory factor analysis and four factors were structured.

The PDI Sinhala version is proved to be valid and reliable tool to assess the burden of psoriasis among Sinhala conversant patients in Sri Lanka.